Revert "pull: Add tls-client-cert-{path,key}"

This reverts commit 94f9ee7bce.
Doesn't actually work, see
https://bugzilla.gnome.org/show_bug.cgi?id=334021

src/libostree/ostree-fetcher.h

@@ -54,9 +54,6 @@ GType   ostree_fetcher_get_type (void) G_GNUC_CONST;
 OstreeFetcher *ostree_fetcher_new (GFile                     *tmpdir,
                                    OstreeFetcherConfigFlags   flags);
 
-void ostree_fetcher_set_client_cert (OstreeFetcher *fetcher,
-                                     GTlsCertificate *cert);
-
 char * ostree_fetcher_query_state_text (OstreeFetcher              *self);
 
 guint64 ostree_fetcher_bytes_transferred (OstreeFetcher       *self);